The chemical spill was the largest single event the West Virginia poison centre has encountered in 20 years.

By 4pm ET on Friday, some 671 residents had called in to the poison centre with concerns on a wide range of symptoms from chemical exposure, the centre's director, Elizabeth Scharman, told The Guardian.

There were also 54 cases of animal exposure.

“The number continues to change every minute,” Scharman said.

She said about half of the callers were reporting symptoms of exposure. “They are complaining of some nausea, vomiting, some dizziness, headaches, diarrhoea, reddening skin, itches and rashes," she said. “There are different complains from a little bit annoying to a little bit more significant.”

A small number of callers were directed to hospital emergency rooms.

The chemical spill was believed to have contaminated the public water supply for 300,000 people, and West Virginians were told on Friday they would not be allowed to drink, bathe, or even wash their clothes in tap water for an indefinite period.

The White House issued a federal disaster declaration for the state, and the US attorney in West Virginia said federal authorities were opening an investigation into the causes of the spill.

The chemical leak at Freedom Industries was discovered by inspectors from the state department of environmental protection on Thursday morning, after residents of Charleston and surrounding areas complained of a strange, sweet smell.

The company did not formally report the spill, of a chemical used to process coal, until about an hour after the inspectors' visit, the Department of Environmental Protection said.

The plant lies just one mile upstream on the Elk River from a water treatment plant.

But it was not required to undergo environment inspection, Tom Aluise, a spokesman for the West Virginia department of environmental protection, told The Guardian.

“The facility is strictly a storage facility, with storage tanks. There is nothing made there. There is nothing processed there,” he said. “We don't inspect that.”

Officials were still trying to determine when the leak started. But Aluise said inspectors were confident that only 5,000 gallons from a 40,000 gallon holding tank had escaped.

“Five-thousand gallons got out of the tank,” he said. “Some of it was contained in a secondary containment area, and some of it got into the river. But we don't know how much got into the river.”

Freedom Industries said in a statement it was “working around the clock” to contain the leak.

The chemical involved, 4-Methylcyclohexane Methanol (MCHM), is an industrial chemical used locally to clean up fine coal dust.

It is not believed to be strongly toxic – in relation to other industrial chemicals – but it is very difficult to clean up. It is also not among those chemicals a water treatment plant would typically test for – making detection much more complicated.

The chemical is also not subject to federal government regulation as a hazardous material.

Environmental groups said on Friday the water crisis in West Virginia was a reminder of a need for greater oversight of industry in the state.

“There is no timeline of when people will have safe water through their taps, and there are still a lot of unanswered questions,” said Angie Rosser, executive director of West Virginia rivers coalition.

“This is an example of the true costs of minimising or ignoring the importance of protective policies.”

One month after the Freedom Industries chemical spill, the water in this house still smells noxious. 'Nothing they could say would ever convince me that this water's okay,' said resident Candase Elswick